David Cameron has now confirmed the RAF killed fellow IS fighter Reyaad Khan, from Cardiff, in an earlier attack on August 21.

Hussain, who tweeted under the alias Abu Hussain al-Britani, escaped to Syria two years ago while on police bail.

Reyaad Khan is also believed to have travelled to Syria in late 2013, and appeared in a video calling for Westerners to fight in Syria and Iraq.

Mr Cameron told the Commons that police had stopped at least six attempts to attack Britain in the last 12 months and added the action against the British men was legally justifiable.

He admitted that the assassination of UK citizens in a foreign country was unprecedented in “modern times”. But he said he would consider similar targeted strikes in Syria and Libya “if there is a direct threat to the British people”.

He added: “If there is a direct threat to the British people and we are able to stop it by taking immediate action then as Prime Minister I will always be prepared to take that action and that’s the case whether the threat is emanating from Libya, Syria or from anywhere else.

“Both Junaid Hussain and Reyaad Khan were British nationals based in Syria who were involved in actively recruiting ISIL sympathisers.

“They were involved in plots to attack ‘high-profile commemorations’ over the summer. We should be under no illusion. Their intention was the murder of British citizens.

“In an act of self defence and after meticulous planning Reyaad Khan was killed in an airstrike on August 21.

“Mr Speaker, we took this action because there was no alternative.”

Mr Cameron said two IS fighters died with Hussain, including another British man, Ruhul Amin.

He added: “The action we took was entirely lawful. We were exercising the UK’s inherent right to self-defence.”

The Prime Minister said the attorney general was consulted and said Britain would be writing to the UN with details.

He added: “This government does not for one moment take these decisions lightly. I do not plan to stand here in the aftermath of a terrorist attack on our streets.”

Junaid Hussain was jailed in 2012 for stealing personal information of Tony Blair and posting it online. He had also targeted America and has posted threatening messages and propaganda videos.

US officials believe he was behind the online radicalisation of at least one of the two gunmen who opened fire at a Prophet Mohammed cartoon competition in Garland, Texas, in May.

Hussain was reportedly in contact with one of the ‘lone wolf’ gunmen before the attack, and tweeted congratulations to his ‘Muslim brothers’ within minutes of the first shots fired.